Use ASN1_INTEGER to encode/decode BIGNUM_it.

Use ASN1_INTEGER to encode/decode BIGNUM_it.

The current code simply shoves the unvalidated ASN.1 bytes into a BIGNUM on
the hope that other things will detect issues (such as negative values
being flipped to positive). Instead of doing this, decode and validate the
ASN.1 data using ASN1_INTEGER, then convert it to a BIGNUM. Similarly, for
encoding convert from BIGNUM to ASN1_INTEGER and use ASN1_INTEGER encoding.

ok tb@